1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy. More particularly, this invention relates to an alkali metal/solid cathode electrochemical cell having reduced voltage delay and irreversible Rdc growth. A preferred couple is a lithium/silver vanadium oxide (Li/SVO) cell. In such cells, voltage delay and permanent or irreversible Rdc growth typically occur from about 25% to about 70% depth-of-discharges (DoD). According to the present inventions, these phenomena are limited by the use of relatively low surface area cathode active materials. The low surface area active material is preferably provided in a free-standing sheet form.
2. Description of Related Art
Voltage delay is a phenomenon typically exhibited in an alkali metal/solid cathode cell, such as of the lithium/silver vanadium oxide couple (Li/SVO) that has been depleted of about 25% to 70% of its capacity and that is subjected to high current pulse discharge applications. It is theorized that in a Li/SVO cell, vanadium compounds become soluble in the cell electrolyte from the cathode and are subsequently deposited onto the lithium anode surface. The resulting anode surface passivation film provides additional resistance, which leads to cell polarization.
The voltage response of a cell that does not exhibit voltage delay during the application of a short duration pulse or pulse train has distinct features. First, the cell potential decreases throughout the application of the pulse until it reaches a minimum at the end of the pulse, and second, the minimum potential of the first pulse in a series of pulses is higher than the minimum potential of the last pulse. FIG. 1 is a graph showing an illustrative discharge curve 10 as a typical or “ideal” waveform of a cell during the application of a series of pulses as a pulse train that does not exhibit voltage delay.
On the other hand, the voltage response of a cell that exhibits voltage delay during the application of a short duration pulse or during a pulse train can take one or both of two forms. One form is that the leading edge potential of the first pulse is lower than the end edge potential of the first pulse. In other words, the voltage of the cell at the instant the first pulse is applied is lower than the voltage of the cell immediately before the first pulse is removed. The second form of voltage delay is that the minimum potential of the first pulse is lower than the minimum potential of the last pulse when a series of pulses have been applied. FIG. 2 is a graph showing an illustrative discharge curve 12 as the voltage waveform of a cell that exhibits both forms of voltage delay.
Decreased discharge voltages and the existence of voltage delay are undesirable characteristics of a pulse dischargeable lithium/solid cathode cell, such as a Li/SVO cell, in terms of their influence on devices such as implantable medical devices including pacemakers and automatic implantable cardiac defibrillators. Depressed discharge voltages and voltage delay are undesirable because they limit the effectiveness and even the proper functioning of both the cell and the associated electrically powered device under current pulse discharge conditions.
Heretofore, a number of patents have disclosed Li/SVO cells and various reforming methods and algorithms to minimize irreversible Rdc growth and voltage delay. For example, U.S. Pat. No. 6,982,543 to Syracuse et al., which is assigned to the assignee of the present invention and incorporated herein by reference, describes methodologies for accurately determining the precise boundaries of irreversible Rdc growth and voltage delay in the about 25% to about 70% DoD region of a Li/SVO cell. This is so that more frequent pulse discharging for the purpose of cell reform is confined to the limits of the region.
Additionally, U.S. Pat. No. 6,930,468 to Syracuse et al., which is assigned to the assignee of the present invention and incorporated herein by reference, describes methodologies for minimizing the occurrence of irreversible Rdc growth and voltage delay in the about 25% to about 70% DoD region by subjecting Li/SVO cells to novel discharge regimes. An optimum discharge regime for a particular cell configuration and electrode material set is determined by subjecting groups of exemplary cells of a particular configuration and material set to a range of different discharge regimes to determine their affects on cell performance.
Additionally, U.S. Pat. No. 7,026,791 to Palazzo et al., which is assigned to the assignee of the present invention and incorporated herein by reference, describes conditioning methodologies for minimizing the occurrence of irreversible Rdc growth and voltage delay in the about 35% to about 70% DoD region by subjecting Li/SVO cells to alternative novel discharge regimes consisting of relatively short high current pulses separated by a relatively short rest period between pulses.
With these methodologies, energy consumption for cell reforming may be a significant portion of the overall discharge capacity. For example, in the embodiments disclosed in the '791 patent of Palazzo et al., up to about 10% DoD may be consumed in cell reforming.
Therefore, there remains a need for a lithium/silver vanadium oxide cell that is dischargeable to deliver the high capacity needed for powering implantable medical devices and the like, but that experiences little, if any, irreversible Rdc growth and voltage delay during pulse discharging, especially at about 25% to about 70% DoD. It is preferable that such a cell does not require the use of a complex discharge regime for cell reforming, nor the process control capability to detect the onset of Rdc growth and then initiate such a discharge regime. In other words, there is a need for a cell with minimal irreversible Rdc growth and voltage delay that is attained solely by the choice of electrode active materials and structures, rather than by the use of complex and power consuming discharge regimes.